Frequency modulation tuning indicator



Nov. 16, 1943. w. l.. CARLSON FREQUENCY MODULATION TUNING INDICATORFiled Aug. '7, 1941 Patented Nov. 16, 1943 FREQUENCY MODULATION TUNINGINDICATOR Wendell L. Carlson, Haddonfield, N. J., assignor to i RadioCorporation of America, a corporation of Delaware Application August 7,1941, Serial No. 405,742 A (ci. '25o-4o) J 4 Claims.

'This invention relates to a frequency modulationtuning indicator forsignal receiving systems, of the frequency'modulation type.

It is an object of this invention to provide an improved tuningindicator for frequency modulation signal receiving systems whichv isresponsiveto resonance and olf-resonance-tuning and which at the sametime indicates the presence or"ab sence of a carrier wave or meanfrequency signal, whereby the tuning indicatormay Vdistinguish betweensignal in tune resonance, and zero `signal and off-resonance tuning.

As is well known, in frequency modulation receiving systems, thediscriminatornetwork, and detector provide audio frequency signal outputfrom the frequency modulated signal and at the same time a directcurrent biasing potential or D. C. component which varies in amplitudeand polarity with variations in tuning above` and below a predeterminedresonance frequency.

This D. C. potential is utilized ordinarily to operate a polarizedindicating device such as a zero-center meter which, however, is subjectvto the disadvantage that the meter reads zero or center for a conditionof exact tuning to resonance and also in the absence of a receivedsignal.l K It is therefore a further object of this invention to providean improved tuning indicator for frequency modulation signals foraffecting two different kinds of visual'indication in one unit comnprising a response to variations in the AStrength of a mean frequency orcarrier signal and a response to the presence of a received signal orthe frequency of the received signal. Y

`The invention will, however, be further understood from the followingdescription, when con- .sidered in connection with the accompanyingdrawing, and its scope is pointed out in the .appended claims.

In the drawing, Figure 1 is a schematic circuit diagram of a portion ofa frequency modulation signal receiving system including a frequencyldiscriminator network provided with` atuning :indicator embodying theinvention, and

Figure 2 is a similar schematic circuitdiagram :showing a modificationof the circuit of Fig. 1 .and also a modified form of tuning indicatorembodying the invention.

- Referring to Fig. 1, a source of frequency modu-A lation signals suchas an intermediate frequency signal circuit 5, is coupled to a seconddetector 'comprising rectiers 6 and 'I through a frequency discriminatornetwork 8 comprising a secondary circuit 9 connected in push-pull to thedetectors 6 and 'I and a primary circuit I0 coupled to said rectiiiersin parallel.

The discriminator and detector output circuit for audio frequencysignals and the D. C. signal component comprises a resistor Il connectedbetween the rectifier cathodes, that is, between the cathode of the tube6 and ground I 2 and an audio frequencyv output circuit I3. This circuitarrangement represents any suitable second detector for a frequencymodulation signal receiving system.

Audio frequency signals from the circuit I3 are applied to an audiofrequency amplifier Ill and to the usual loudspeaker I5 which terminatesthe signal channel of the receiving system.

The D. C. component of the detected signal which varies in amplitude andpolarity with tuning is utilized to actuate a portion of the tuningindicating means, which in the present'example comprises apolarizedmilliammeter element I6 having a light pointerV I'I which moves over azerocenter graduated scale I8 formed on kan anode or positive plateelectrode I9 of an indicator tube 20. This is included in a D. C. pathin shunt with the output impedance II from the ground terminal of thediscriminator output impedance Il through a lead 2l and with the highpotential side of the output impedance II at the circuit I3 through alead 22 and a decoupling resistor 23.

In the absence of signals, the meter I6 reads zero, or center scale.Likewise at resonance withA a received signal it reads zero or centerscaie, for the reason that the D. C. potentials at the terminals of theoutput impedance I l are equal and opposite. For conditions ofolf-,resonance tuning the meter will read to the right or left of zerodepending upon whether the tuning is above or below resonance and willtherefore indicatethe direction for tuning to correct the off-resonancecondition.

The` presence or absence of a received mean frequency or carrier signalis indicated by the same visual indicating element comprising thepointer Il and the plate I9. VFor this purpose the latter is coated witha material such as willemite which becomes fluorescent when bombarded byelectrons. The electron source in the present example is the cathode 25of the tube 28. i

The plate I9 is 4connected through a lead 26 with a positive terminal 21on a potential supply l source such as a potentiometer 28 to which thecathode is connected at a more negative point 29. The electron iiow fromthe cathode to the plate or anode I9 is controlled by a control grid 30interposed between the cathode and the plate as indicated. The grid isoperated at a negative potential supplied through a grid resistor 3|from a negative terminal 32 on the supply source 28. The grid isactuated or placed under the control of the incoming signal through aconnection 33 with the input circuit I0. A coupling capacitor 34 intheconnection 33 servesto isolate thegrid 30 from D. C. potentialsappearing in'the circuit which may be connected to an intermediate fre--quency amplifier or the like (not shown).

The bias potential between the cathode connection 29 and the gridconnection 32 on the supply source 28 is such that the electron flow oranode current of the tube 20 is substantially-Zemin the absence of areceived mean frequency signal.

that it may be applied to frequency modulationl circuits vwhich providea voltage which varies in magnitude and polarity in yresponseftovariation from resonance of a received frequency modula- Ationsignal, that is, proportional to the frequency of an applied signal, anda signal voltage as it 7 varies in intensity or is absent, that is, thestrength Therefore the scale I8 is not fluorescent in the l absence ofsignals. When the signalis'tuned in,

the pointer I1 gradually approaches zero and the scale becomesincreasingly fluorescent or illuminated as the strength of the receivedsignal Aincreases in the circuit l0 and on the control grid 30. With onevisual indicating device, having two different indicating elements whichmaybe viewed/at the same time when operating the receiving system, thepresence or kabsence of a received signal is indicated, and the`resonance indication in the presence of a received signal differs fromthe indication provided in the absence of a received signal. c

Referring now to Fig. 2 wherein the samereference numerals refer to likeparts as in Fig. l, the D. C. control voltage from the discriminatoroutput circuit Il is applied Yto the control grid 40 of a D. C.amplifier 4l which is coupled across an output resistor 42 to anelectron beam deiiector plate 43 in a tuning indicator tube 44. A seconddeiiector plate is provided on the opposite side of the tube which isindicated at 45'. rIhe beam 46 is between a cathode 41 and an anode 48which bears a scale 49 adapted to become fiuorescent in the path of thebeam which moves across the scale 49 under control of the electrodes 43and 45. A positive potential is placed on the electrode 45 through aconnection 50 c with a source of anode potential 5I having a groundednegative terminal 52. The anode 4B is also energized from the samesource through va connection lead indicated at 53, the cathode 1l-beingconnected to ground 54.

The strength of the electron beam is controlled by a control grid 55interposed between the cathode 4lY and the anode 48 and connected tocathode and ground through a suitable source of `biasing potential 56and a grid resistor 51. Signals are supplied from the input circuit 5tothe grid 55 through the lsignal supply circuit 33 and the couplingcapacitor 34.

`With this arrangement,-1n the absence of re-` ceivedf signals, .the.grid `55 is biased negativelylby the source 56, preventing the flow ofany substantial amount of plate current and resultingA in an electronbeam which .is linsuflicient to cause iiuorescense of the scalev49.'As-asignal is tuned in,vthe beam moves over :the scale, increasing instrength until it becomes a iiuorescent moving spot on the scale whichcomes -toa central position` upon exacttuningto resonance. Thus,y twocontrol functions for the indication of .resonance are provided in asingle,rvisual indicator, whereby for frequency .modulation `reception.fan `-oltxerator may distinguish between a received signal l.tuned toresonance and an off-resonance .indication of a received signal, as well.as a vcondition .provided Y ofthe applied signal, to provide the secondcontrolling function in the indicator.

vI claim as" my invention:

1. A frequency modulation tuning indicator comprising a source ofillumination, means for varying the intensity of illumination derivedfrom said source in proportion to the strength of an applied signal, amovable indicator within A the iield of illumination of said source, andmeans lfor varying the position of said indicator in saidlield ofillumination proportional to the frequencyof the applied signal. .l

2. A frequency vmodulation tuning indicator comprisinga signal circuit,a source of illumina-A ticn,. a scale within the field of illuminationof said source, means forvarying the intensity of illumination on saidscale in proportion to Ythc strength of an applied signal through saidcircuit, a movable indicator associated with said scale within the fieldof illuminationof said source, and means connected' with said circuitfor varying the position of said indicator onsaid scale vpropor-v tionaltothe frequency'variation of the applied signal with respectto apredetermined resonance frequency. c

3. A' 'frequency modulation tuning indicator comprising in combinatiomanelectric discharge tube having a control grid, a cathode and an .an-lode, means providing an indicator scale on .said anode which becomesvisibly fluorescent in response to Va. predetermined electron ow throughsaid tube underI control of said grid, means for applying a receivedsignalto said grid, a frequency discriminator network responsive toreceived' signals .and having an output circuit providing a controlpotential which varies in magnitude and polarity with variation infrequency of an applied signal from resonance, and means responsive tosaid voltage for indicating thefrequency variation of said appliedsignal on said indicator scale.

4. In a frequencymodulation signal receiving system, a frequencymodulation tuning indicator comprising -a frequency Ymodulation signalcircuit,anV electron discharge tube having a control grid coupled tosaid circuit, Ia cathode and anv an-v ode, means providing an indicatorAscale on said anode which -fluoresces in response to the flow ofelectrons theretofunder'con-t-rol -of said gridv as an indication ofthesignal strength, a frequency discriminator ynetworkcoupled to saidcircuit and having an output circuit providing a control potential whichvaries in -magnitude and polarity proportionalto the frequency variationof an' applied .signal from the resonance frequency ofthe network, andmeans responsive to-saidpotential for imparting tothe vscale 4a vsecond`indication proportional to the frequency variation of anapplied signalfrom said resonance frequency.

WENDELL .n CARLSON. f f

